The present invention relates generally to electro-optical interfaces and more specifically to a self-powered electro-optical interface.
Serial interfaces, and particularly serial interfaces conforming to the RS-232C standard are a popular interface between personal computers and remote equipment. This equipment includes modems, mice, printers and more recently, various types of measuring instruments. One problem which arises when a computer is used with a measuring instrument, especially an instrument which measures low levels or sensitive signal levels, is noise. Under extreme circumstances, noise may completely mask the measured signal or cause overload in the measuring instrument. It is common for noise to be development due to ground loops. A ground loop occurs when a voltage or potential difference exists between the ground at the instrument and ground at the computer. It is possible, for instance, to connect the instrument to one ground point and a computer to another, such as is possible if the two are not in proximity to each other.
Further, if large electrical loads are present on the building's electrical system, the local grounds will be at slightly different electrical potentials. Since the RS-232 interface requires a signal common or ground to operate, a current will flow along the interface ground wire. The building's ground wire will act as both a return path and as a coupling medium. Since the building ground wire is commonly run alongside the wires carrying current to the large load, magnetic fields will couple energy into the ground wire much the same way as a transformer. Problems which will arise include signal degradation or erratic interface operation.
One common way to reduce noise under these circumstances is to couple the interface signals over a device which allows for the galvanic separation of two ground systems. One common device is an opto-coupler. The use of an opto-coupler has heretofore required external power to operate the devices and any other required interface circuitry. This has previously required the added expense of a transformer or batteries. Some examples of prior art devices are described hereafter.
U.S. Pat. No. 4,161,650 to Caouette, et al. shows a self-powered fiber optic interconnected system. The circuitry in FIG. 3 derives its power for the interface from the incoming electrical signals and transmits this electrical power over line 45 to the other interface in parallel to the optically transmitted signals.
U.S. Pat. No. 4,417,099 to Pierce shows an electro-optical isolator circuit for line powered modems. Multiplexer 32, interface 20 and data buffer 18 each include the electro-optical isolator of FIG. 2. The telephone line interface and power device 10 derives the operating power from the telephone line to all modem circuits.
U.S. Pat. No. 4,420,841 to Dudash shows optically coupled half duplex bidirectional transceiver. A test instrument 12 is isolated from micro-processor based encoder 20 by optical isolators 16, 18; 24, 38; and 30, 32. Separate power sources V1, V2 and V3 are provided to various portions of the transceiver 110.
U.S. Pat. No. 4,485,439 to Rothstein shows a sophisticated interface circuit for connecting an instrument with a host digital computer. The instrument 102 may provide data in various physical formats such as synchronous or asynchronous RS-232, or passive or active current loop, or discrete digital TTL signals. The three types of physical compatibility translator elements 300, 302 and 304 are illustrated for the V.24 RS-232 mode, current loop active mode and current loop passive mode respectively. FIGS. 4 and 5 show electro-optical isolators. A power source 126 is shown in the interface device.
U.S. Pat. No. 4,639,727 to Blasius, et al. shows optical couplers for interfacing RS-232s .+-.12 volts over cable 2 with an RS422 0 to +5 volts over wire 4.
U.S. Pat. No. 5,121,491 to Sloan, et al. shows an interface between MIDI to RS-232 using electro-optical couplers. Power convertor 37 receives the data terminal release DTR signal from pin 20 of the host computer 9 and uses it as a source of power and for power regulation. This allows the 5 volt signal from the MIDI to be provided to an appropriate level for the computer 9 based on the signal level at DTR Pin 20. The signal is also used to enable and disable the interface.
U.S. Pat. No. 5,157,769 to Eppley, et al. shows an interface between a host computer and a second computer. The interface is designed to interface a 3-6 volts system and a .+-.12 volts system. As illustrated in FIG. 3, the power for the interface is from data out pin 2 of the host computer. An optical isolator 52 activates and deactivates the ground pin of the smaller device. Thus, the smaller device is not powered-up unless there is a signal on terminal 20 of the host computer. Amplifier 40 provides the interface from the smaller device to the host computer, and transistor 44 provides the interface between the host computer and the smaller device.
These previous devices have not provided an inexpensive interface which is readily available to many situations for example interconnecting a computer having its own power source with a device also having some power source.
Thus it is an object of the present invention to provide an inexpensive isolating interface between two devices wherein one of the devices is a computer.
Another object of the present invention is to provide an isolating interface which is capable of being incorporated into a device which communicates with a computer.
A further object of the present invention is to provide an interface with no independent power source which is capable of interfacing a RS-232 buss with a TTL level buss.
These and other objects are achieved by providing two optical isolators in an interface which produce bidirectional communication interfaces between first and second devices. The first device provides positive, negative and neutral ports to its portion of the interface. The second device provides first and second power ports for its portion of the interface. The power ports are independent of the signals or data inputs and outputs of the interface and are used to power their portion of the interface and define the two levels of signals necessary at each output of the interface. Capacitors are provided between the positive, negative and neutral ports for establishing reference values. The positive and negative ports of the interface are from the first device which preferably is a computer and are signals provided on unused data or control signal ports. The interface may be incorporated into the second or non-computer device.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.